Rumen undegradable protein (RUP) and its intestinal digestibility after steam flaking of cereal grains

While it is known that heat treatment of cereal grains generally improves the nutritional value for ruminants, simultaneous information on rumen degradability and intestinal digestibility of the rumen by-pass is scarce, especially for non-starch constituents. The effect of steam flaking at 90°C for 30 min on protein quality of maize, wheat, and barley was studied. In addition to proximal chemical analyses, protein rumen degradability was determined in vitro and intestinal digestibility of rumen undegraded protein was determined using the mobile bag method. No significant effects of steam flaking on chemical composition of cereal grains (crude protein, acid detergent fibre, neutral detergent fibre, and starch) were observed. The protein fractions that are relevant to rumen degradability were significantly influenced by the steam flaking: the non-protein nitrogen fraction (A) was reduced (P < 0.05) by 43-93% for all three cereal grains, whereas the fraction that represents true soluble protein (B1) was reduced (P < 0.05) for wheat grains (by 77%) and barley (by 93%). Although the difference was not significant, a decrease of 41% for B1 fraction was also observed for maize. On the other hand, steam flaking markedly increased buffer insoluble but neutral detergent soluble protein fraction (B2) by 15-25% for all three cereal grains, whereas effects on B3 fraction were not significant. Steam flaking was also associated with an increase of the rumen undegradable protein fraction (C) in the case of barley, from 6.6 to 11.1% of total N (P < 0.05) and wheat, from 5.2 to 8.4% of the total N (P < 0.05). Overall, the changes of the protein fractions led to a considerable increase of the rumen undegradable protein (RUP) for all steam-flaked cereal grains (P < 0.05). Steam flaking also increased (P < 0.05) intestinal digestibility of RUP of the cereal grains (by 3.6-34.8%), leading to a more efficient protein use in ruminants.